Offset compensated sensing for magnetic random access memory

ABSTRACT

An offset compensated memory element voltage supply including a differential amplifier with a compensation circuit, and a transistor with a gate connected to the output of the differential amplifier. The compensation circuit of the differential amplifier includes a compensation capacitor that stores a compensation voltage during a calibration phase, and applies the stored compensation voltage to a compensation input of the compensation circuit of the amplifier during a measurement phase. Feedback from a source of the transistor controls the output of the differential amplifier to maintain a standard voltage across a resistive memory element connected to the source during measurement of the resistance of the resistive memory element, and the compensation circuit improves the accuracy of the voltage across the resistive memory element by compensating for an offset voltage of the differential amplifier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of resistor-based memorycircuits. More particularly, it relates to a method for preciselyregulating the voltage applied across a resistive memory element duringsensing.

2. Description of the Related Art

FIG. 1 shows one example of a resistor based memory. The memory includesa memory cell array 8 having a plurality of row lines 10 arranged innormal orientation to a plurality of column lines 12. Each row line isconnected to each of the column lines by a respective resistor 14.

A Magnetic Random Access Memory (MRAM) is one approach to implementing aresistor based memory. In an MRAM, each resistive memory cell includes amagnetizable film. The resistance of the cell varies, depending on themagnetization state of the film. Logical data can be stored bymagnetizing the film of particular cells so as to represent the logicstates of the data. One resistance value, e.g., the higher value, may beused to signify a logic “HIGH” while the other resistance value, e.g.,the lower value, may be used to signify a logic “LOW”. The stored datacan be read by measuring the resistance of the cells, and interpretingthe resistance values thus measured as logic states of the data.

For MRAM sensing purposes, the absolute magnitude of resistance need notbe known; only whether the resistance is above or below a value that isintermediate to the logic high and logic low values. Nonetheless sensingthe logic state of an MRAM memory element is difficult because thetechnology of the MRAM device imposes multiple constraints. In a typicalMRAM device an element in a high resistance state has a resistance ofabout 1 MΩ. An element in a low resistance state has a resistance ofabout 950 KΩ. The differential resistance between a logic one and alogic zero is thus about 50 KΩ, or 5% of scale. Rapidly distinguishing a5% resistance differential on a scale of 1 MΩ with a minimum ofcircuitry is problematic.

Resistance is measured using Ohm's Law which holds that resistance isequal to the voltage across a resistor divided by the current throughthe resistor. Generally one parameter is held constant while the otheris measured. In MRAM applications voltage is typically held constantwhile current is measured. As a result, the quality of an MRAMresistance measurement depends in large part upon the ability toregulate the voltage applied across a resistive memory element duringsensing.

Where a stabilized voltage is required, it is known to use a voltagefollower circuit constructed by feeding back an output signal of adifferential amplifier to an inverting input of the amplifier, andconnecting a reference voltage to the non-inverting input of theamplifier. FIG. 2 shows one embodiment of a conventional voltagefollower in block diagram form. A voltage follower exhibits a voltagegain of about 1, high input impedance, and low output impedance. Avoltage follower circuit might thus be used to apply a voltage across aresistive memory element to sense the resistance of the resistor.

In practice, a conventional differential amplifier, such as might befound in a memory device, has offsets that affect the voltage across thememory cell. If an amplifier with an offset is used to provide a voltageacross a MRAM resistive memory element for sensing purposes, thepresence of the voltage offset in the differential amplifier reduces theaccuracy of the voltage applied across the memory cell, and consequentlythe precision with which the logic state of the cell is read. Therefore,there is a need for an improved voltage source circuit that is readilyimplemented on an integrated circuit, that requires few components, thatoperates at high speeds, and that provides a highly accurate and stablevoltage output and a current output appropriate to support MRAM memoryelement sensing.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for measuring theresistance of a resistive memory element. The resistance is measured byapplying a controlled voltage across a resistive memory element andascertaining the level of current that consequently flows through theresistive memory element. According to the present invention, animproved differential amplifier, including a compensation circuit, isused in a feedback configuration to set a standard voltage across aresistive memory element. The compensation circuit reduces the effect ofdifferential amplifier offset and yields a more precise standard voltageacross the resistive memory element.

In one aspect, the invention includes a memory element voltage supplyincluding a transistor with a gate adapted to control an electricalcurrent through the transistor in relation to a voltage applied to thegate. The transistor also includes a source (or drain depending ondevice polarity) operatively connected to the resistive memory device.The voltage supply includes a differential amplifier with anon-inverting input operatively connected to a source of a referencevoltage, an inverting input, a first output switchingly connected to thegate of the transistor, and a second output. The supply further includesa compensation amplifier with a third input adapted to receive and storea first compensation voltage, a third output operatively connected tothe first output, and a fourth output operatively connected to thesecond output. Also included is a feedback line including a switchadapted to switchingly connect the second input to the source (or drain)of the transistor.

In operation the voltage supply first enters a calibration phase. Duringthe calibration phase, a first compensation voltage is stored at thethird input of the compensation amplifier. Generally, this third inputincludes a capacitor which is charged to store the first compensationvoltage. The compensation voltage stored reflects the magnitude of anoffset voltage that exists in the differential amplifier when nodifferential voltage is applied across its inputs; that is, thecompensation voltage reflects an error that exists in the output of thedifferential amplifier. After the calibration phase, the voltage supplyenters a measurement phase. In the measurement phase the error voltage,stored on the capacitor, is used as an input to the compensationamplifier. The resulting output of the compensation amplifiercounteracts the erroneous offset voltage that would otherwise beincluded in the output of the differential amplifier. As a result ofthis corrective action, the output of the differential amplifier is moreaccurate, and a more precise measurement may be made of the resistanceof the resistive memory element than would otherwise be possible.

These and other aspects and features of the invention will be moreclearly understood from the following detailed description which isprovided in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional magnetic random access memory array inelectrical schematic form;

FIG. 2 shows a conventional voltage follower circuit including adifferential amplifier in block diagram form;

FIG. 3 shows a portion of a magnetic random access memory deviceaccording to one aspect of the invention in electrical schematic form;

FIG. 4 shows a portion of a magnetic random access memory deviceaccording to one aspect of the invention including a sensing circuit andsneak resistance;

FIG. 5 shows a conventional voltage follower circuit including adifferential amplifier and a transistor for applying a sensing voltageto a resistive memory element;

FIG. 6 shows a conventional circuit for a differential amplifier inelectrical schematic form;

FIG. 7 shows a circuit for a differential amplifier according to oneaspect of the invention including components for offset voltagecompensation;

FIG. 8 shows, in block diagram form, a circuit according to one aspectof the invention for applying a standard voltage across a resistivememory element;

FIG. 9 shows, in block diagram form, the configuration of a circuitaccording to one aspect of the invention during calibration phase;

FIG. 10 shows, in block diagram form, a configuration of a circuitaccording to one aspect of the invention during measurement phase;

FIG. 11 shows, in block diagram form, a computer system incorporating adigital memory device including a compensated voltage source accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows a resistive memory device according to one aspect of theinvention. In the embodiments illustrated herein the resistive memory isreferred to as MRAM memory; however, the invention is applicable to anytype of resistance memory and is not limited to MRAM memory. The deviceincludes an array 16 of Magnetic Random Access Memory (MRAM) elements, aplurality of electrically conductive row lines 10, and a plurality ofelectrically conductive column lines 12. Each row line is connected toeach of the plurality of column lines by a respective MRAM resistiveelement 14. A plurality of switches 18, typically implemented astransistors, are each switchingly connected between one of the row linesand a first source of constant potential (ground) 20. A plurality ofsensing circuits 22 are respectively connected to the plurality ofcolumn lines 12. Each sensing circuit 22 includes a source of constantelectrical potential (V_(A)). According to the invention, the precisionof this source of constant electrical potential (V_(A)) is improved witha compensation circuit.

A separate plurality of pull-up voltage sources 24 are respectivelyconnected to each of the plurality of row lines 10. These supply voltage(V_(A)) to the row lines 10. These pull-up voltage sources 24 serve tomaintain the ungrounded row lines at array voltage (V_(A)), therebypreventing leakage current through so-called sneak resistance.Optionally, the voltage compensation method of the present invention maybe applied to the plurality of pull-up voltage sources 24 attached tothe row lines 10 respectively.

In operation, an exemplary switch 18, such as switch 26 associated witha particular row line 28, is closed so as to bring that row line toground potential and a particular column line 12, for example columnline 30, is used by a respective sensing circuit 32 to read theresistance value of resistor 34.

FIG. 4, shows the resulting electrical circuit for the relevant portion36 of the memory array when row 28 and column 30 are activated. Asshown, memory element 34 is connected between a grounded row line 28 anda particular column line 30. Also connected to the column line 30 are aplurality of other resistive memory elements (e.g., elements 38, 40, 42,44, 46) each of which is connected at its opposite end to a source ofelectrical potential (V_(A)) 24 through a respective row line 10. Inaddition, a respective sensing circuit 32 is connected to the columnline 30. The sensing circuit 32 includes a voltage supply, according tothe present invention, that maintains the column line 30 at electricalpotential (V_(A)). The other resistive memory elements (those tied toungrounded row lines) 38, 40, 42, 44, 46, form an equivalent resistancereferred to earlier as sneak resistance. The effective resistance of thesneak resistance is small. A typical value for sneak resistance might beone KΩ. Nevertheless, because both ends of each ungrounded resistor areideally maintained at the same potential (here V_(A)), net current flowthrough the sneak resistance is desirably nearly zero. Precise voltageregulation across the sneak resistance is therefore important, since anyvoltage differential across the sneak resistance results in an unwantedcurrent flow through the sneak resistance that reduces measurementprecision and increases power consumption and thermal losses. Incontrast, a measurable current desirably flows through the groundedresistive memory element 34. This current allows measurement of theresistance of the memory element 34 by the sensing circuit 32.

FIG. 5 shows a conventional circuit 50 for applying a sensing voltageacross a resistive memory element. A differential amplifier 52 isprovided, including a non-inverting input 54, an inverting input 56, andan output 58. Also shown are supply 60 and ground 62 connections of thedifferential amplifier connected to a source of supply (VCC) 64 and aground 20 respectively (note that in an alternative embodiment, thedifferential amplifier might be connected for power between twonon-ground potentials). The non-inverting input 54 of the differentialamplifier is operatively connected to a source of a reference voltage(V_(A)) 66. The output 58 of the differential amplifier is operativelyconnected to a gate 68 of a transistor 70. As would be understood by oneof skill in the art, transistors of various polarities may be employed,and the output signal of the differential amplifier invertedaccordingly. As shown, an enhancement mode NMOS field effect transistoris used. A drain 78 of the transistor is connected to a power supply(VCC) 64 and a source 82 of the transistor is connected to a column line30 which is, in turn, connected to one end of a resistive memory element34. The other end of the resistive memory element 34 is connected to arow line 28 that is switchingly connected to a ground potential 20 by aswitch 26. The column line 30 is also connected to the inverting input56 of the differential amplifier 52 and to the sneak resistancecollectively indicated as 84. In operation an output signal of thedifferential amplifier controls the gate 68 of the transistor tomaintain a voltage ideally equal to the reference voltage (V_(A)) 66 onthe column line 30.

In practice, offsets within the differential amplifier result in avoltage at the column line 30 that deviates from that of the referencevoltage 66. The offset within the differential amplifier is representedin FIG. 5 by a voltage 86 applied to or subtracted from thenon-inverting input 54 of the differential amplifier.

FIG. 6 shows a schematic representation of a circuit 88 of aconventional differential amplifier used as differential amplifier 52.The circuit includes an output connection 58 an inverting transistor 90including a gate 92 connected to an inverting input 56 and anon-inverting transistor 94 including a gate 96 connected to anon-inverting input 54. The drain 98 of the non-inverting transistor 94is operatively connected to the source of supply voltage (VCC) 64. Thedrain 100 of the inverting transistor 90 is operatively connected to theoutput 58 of the differential amplifier, and also to one end 102 of apull-up resistor 104. The other end 106 of the pull-up resistor isconnected to the source of supply (VCC) 64. First 108 and second 110source connections of the inverting 90 and non-inverting 94 transistorsrespectively are connected in common to a first terminal 112 of acurrent source 114. A second terminal 116 of the current source 114 isconnected to ground 20. Also shown is a voltage source 86 representing,a +/− offset within the differential amplifier 88. Note that althoughoffset voltage is represented throughout the present disclosure as alumped voltage source, this is a simplified representation. Actuallyoffsets may arise from a variety of causes and components distributedwithin the electrical components of the resistance measurement voltagesupply.

FIG. 7 shows a compensated differential amplifier circuit according toone aspect of the invention. The circuit includes an operativeconnection to a source of supply voltage (VCC) 64, a pull-up resistor110 and an output connection 112. The output connection 112 is connectedto one end 114 of the pull-up resistor 110. The other end 116 of thepull-up resistor is operatively connected to the source of supplyvoltage (VCC) 64. The amplifier also includes an inverting transistor118 with a gate 120 connected to an inverting input 122 and anon-inverting transistor 124 including a gate 126 connected to anon-inverting input 128. A drain 130 of the inverting transistor 118 isconnected to the output 112 of the amplifier. A drain 132 of thenon-inverting transistor 124 is operatively connected to the source ofsupply voltage (VCC) 64. Optionally, the drain 132 of the non-invertingtransistor may be available on a complementary output. First 134 andsecond 136 source connections of the inverting 118 and non-inverting 124transistors respectively are connected in common to a first terminal 138of a first current source 140. A second terminal 142 of the firstcurrent source 140 is connected to ground 20. Also shown is a voltagesource 144 representing, a +/− offset within the differential amplifier146. As discussed above, although offset voltage 144 is shown as alumped voltage source, this is a simplified representation. Offsets atthe output of the differential amplifier 146 may result from variouscauses such as component asymmetry within the differential amplifier146.

According to one aspect of the invention, the amplifier 146 includes anoffset compensation circuit 148 in the form of a second differentialamplifier. The offset compensation circuit 148 includes a firstcompensation transistor 150 and a second compensation transistor 152.The drain 154 of the first compensation transistor 150 is operativelyconnected to the drain 130 of the inverting transistor 118, andconsequently to the output 112 of the differential amplifier 146. Thedrain 156 of the second compensation transistor 152 is operativelyconnected to the drain 132 of the non-inverting transistor 124. A sourceconnection 158 of the first compensation transistor 150 and a sourceconnection 160 of the second compensation transistor 152 are connectedin common to a first terminal 162 of a second current source 164. Asecond terminal 166 of the second current source 164 is connected toground 20. A gate 168 of the first compensation transistor 150 isconnected to a first terminal 170 of a first compensation capacitor 172.A second terminal 174 of the first compensation capacitor 172 isconnected to ground 20. A gate 176 of the second compensation transistor152 is connected to a first terminal 178 of a second compensationcapacitor 180. A second terminal 182 of the second compensationcapacitor 180 is connected to ground 20.

FIG. 8 shows an operational connection 200 of the differential amplifierof FIG. 7 according to one aspect of the invention. As shown in FIG. 8,the amplifier 146 of FIG. 7 is represented as an amplifier block 202 andincludes the portion of compensation circuit 148 identified in FIG. 7 as186. The first 172 and second 180 compensation capacitors and theirmutual connection to ground 20, shown in FIG. 7 as portions 184, 188 ofFIG. 7 are shown externally in FIG. 8. According to the invention, theoutput 112 of the differential amplifier 202 is connected through aswitch 204 to a gate 68 of a control transistor 70. The controltransistor includes a drain 78 connected to a power supply (VCC) 64 anda source 82 connected to a column line 30 of a resistive memoryintegrated circuit device. The column line 30 is connected to oneterminal 206 of a resistive memory element 34. The resistive memoryelement includes a second terminal 208 connected to a row line 28 thatis switchingly connected to ground 20 through a switch 26. Thedifferential amplifier 202 includes a non-inverting input 128operatively connected to a source of a reference voltage 66. Theamplifier 202 also includes an inverting input 122 connected by means ofa switch 210 to the same reference voltage (V_(A)) 66. The invertinginput 122 also is switchingly connected by means of a second switch 212to the column line 30. The first terminal 170 of the first compensationcapacitor 172 is also switchingly connected by means of a further switch214 to the output 112 of the differential amplifier 202. The firstterminal 178 of the second compensation capacitor 180 is switchinglyconnected by means of a further switch 216 to the reference voltagesource (V_(A)) 66. As noted above, the second terminals 174 and 182 ofthe first compensation capacitor 172 and of the second compensationcapacitor 180 respectively are connected to ground 20.

A control circuit 218, which may be external to the amplifier 202,controls switches 204, 210, 212, 214, 216 to switch the amplifierbetween a calibration mode configuration, and a resistive elementmeasurement mode configuration. The control circuit 218 includes a firstphase output 220 operatively connected to switches 210, 214, and 216.The control circuit 218 includes a second phase output 222 operativelyconnected to switches 204 and 212.

In operation, the differential amplifier circuit shown in FIG. 8switches between two modes; a calibration mode, or a resistancemeasurement mode.

In a calibration mode, switches 204 and 212 are open; switches 210, 214and 216 are closed. Generally speaking, the switches 204, 210, 212, 214,and 216 (FIG. 8) are implemented as transistors, and are operated underthe control of control circuitry 218. Prior to sensing the stored valueof a resistive memory element, the control circuitry 218 places thedifferential amplifier 202 in a calibration mode by opening switches 204and 212, and closing switches 210, 214, and 216. FIG. 9 shows theresulting connection of the differential amplifier of the invention. Thenon-inverting 128 and inverting 122 inputs of the differential amplifier202 are connected together and held at reference potential (V_(A)) 66.The first terminal 178 of the second capacitor 180 is also held atreference potential (V_(A)) 66. In this configuration, there is nodifferential voltage across the inputs 128, and 122, of the differentialamplifier. Any voltage that appears on the output 112 is therefore anoffset voltage and is, by definition, undesirable. Because the circuitis in a calibration mode configuration, this offset voltage is fed backthrough switch 214 to the first compensation capacitor 172. The firstcompensation capacitor 172 is charged by the differential amplifieruntil the voltage on the capacitor 172 is equal to the offset voltage atthe output 112 of the differential amplifier 146. Thereafter, thecontrol circuitry places the amplifier in the resistance measurementmode. Once the circuit is returned to measurement mode, this storedvoltage acts to produce an inverse offset that negates the offset thatwould otherwise be present at the output 112 of the amplifier 202.

FIG. 10 shows the differential amplifier 202 connected for use in aresistance measurement mode. In resistance measurement mode, switches210, 214, and 216 are open, and switches 204 and 212 are closed. Thereference voltage (V_(A)) 66, plus the offset voltage 144 are applied tothe non-inverting input 128 of the differential amplifier 202. The firstcompensation input 224 corresponds to the gate 168 of the firstcompensation transistor 150. Accordingly, the pre-charged firstcompensation capacitor 172 applies a stored voltage of (V offset) to thegate 168 of the first compensation transistor 150 (FIG. 7). The secondcompensation capacitor 180 exhibits pre-stored charge (V_(A)) that isapplied to a second compensation input 226 of the differentialamplifier. The second compensation input 226 corresponds to the gate 176of the second compensation transistor 152. Accordingly, the pre-storedvoltage (V_(A)) is applied to the gate 176 of the second compensationtransistor 152 (FIG. 7). As a result, the first 150 and second 152compensation transistors shunt current from the drains of the inverting118 and non-inverting 124 transistors respectively to ground 20. Thisshunt current is just enough to adjust the voltage that appears on theoutput 112 of the amplifier 146 so as to substantially eliminate voltageoffset from that output.

The combination of the compensated differential amplifier and thetransistor 70 act to maintain a voltage (V_(A)) on column line 30, andaccordingly across the resistive memory element 34 during measurementwith minimized errors due to offsets in the differential amplifier 202.If the voltage on the column line 30 drops below (V_(A)), negativefeedback into the inverting input 122 of the differential amplifierresults in a signal to the gate 68 of the transistor that increasescurrent flow through the transistor and returns the voltage of thecolumn line 30 to voltage (V_(A)).

In one aspect, the control circuit 218 acts to place the amplifier incalibration mode at the beginning of every read cycle. In a variation,the control circuitry places the amplifier in calibration mode prior toa first read, and then returns the amplifier to calibration mode onlyafter a predefined number of read cycles. In a further variation, thecontrol circuit 218 places the amplifier in calibration mode prior to afirst read cycle, and then returns the amplifier to calibration modeonly after a definite value of voltage offset is sensed on the output112.

FIG. 11 shows a computer system 230 including a memory 232 including oneor more resistive memory devices which are constructed to include theinvention as described and illustrated with reference to FIGS. 7-9. Thecomputer 230, as shown includes a central processing unit (CPU) 234, forexample a microprocessor, that communicates with one or moreinput/output (I/O) devices 236 over a bus 238. The computer system alsoincludes a peripheral device such as disk storage 240 and a userinterface 242. It may be desirable to integrate the processor 234 andmemory 232 on a single integrated circuit device. Other processingsystems in which the invention, in different embodiments may be employedinclude radio systems, television systems, wireless, wired, and opticalvoice and data networks, global positioning system receivers, and othersystems where digital data storage is required.

While preferred embodiments of the invention have been described in theillustrations above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,deletion, substitution, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as limited by theforegoing description but is only limited by the scope of the appendedclaims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A sensing circuit for a resistive memory cellof a memory device, said sensing circuit comprising: a first circuit forholding a column line connected to said memory cell at a predeterminedvoltage during a resistance measuring operation; and a compensationcircuit coupled to said first circuit for compensating for voltageoffsets inherent in said first circuit.
 2. A sensing circuit as in claim1, wherein said first circuit comprises a first differential amplifiercircuit.
 3. A sensing circuit as in claim 2, wherein said firstdifferential amplifier includes a non-inverting input operativelyconnected to a reference voltage, and an inverting input switchinglyconnected to said column line.
 4. A sensing circuit as in claim 1,wherein said compensation circuit comprises a second differentialamplifier circuit.
 5. A sensing circuit as in claim 1, wherein saidcompensation circuit operates during a calibration mode period to detectand store offsets in said first circuit and uses said stored offsets toadjust an output of said first circuit during said resistance measuringoperation.
 6. A sensing circuit as in claim 5, wherein said storedoffsets further comprise a first stored reference value and a secondstored output value.
 7. A sensing circuit as in claim 6 wherein saidcompensation circuit further comprises first and second capacitorsadapted to store said first stored reference value and second storedoutput value respectively.
 8. A sensing circuit as in claim 1 whereinsaid first circuit includes first and second inputs, said second circuitincludes third and fourth inputs, and wherein during a calibration modetime period said first, second, and fourth inputs are mutually connectedto a first reference voltage source, and said third input is connectedto an output of said first circuit.
 9. A sensing circuit as in claim 8,wherein during a sensing mode time period said first input is connectedto said first reference voltage source, and said second input isconnected to said column line.